This invention relates generally to furnace controls and more specifically to microprocessor based gas furnace controls.
Typically, the control of gas furnaces includes the control of main and induced draft fan motors having selected time delays in conjunction with an ignition control, gas valve and thermostat.
Control of these functions by a microprocessor is known; however, such controls have suffered from the limitation that their timing mechanisms have been more erratic than desirable. Utilizing IC networks such as internal oscillator for timing results in an unsatisfactory tolerance with timing varying plus or minus fifty percent or more. Not only does the timing vary within a particular microprocessor but also from one microprocessor to another. There is a need to provide a control which has significantly improved reliability, particularly in relations to providing consistent timing functions over a wide temperature range, e.g., from minus 40.degree. C. To 85.degree. C.
It is an object of the present invention to provide a control for gas furnace controls which has improved, consistent and reliable timing.
Another object is the provision of a microprocessor control which has timing consistency within plus or minus ten percent over a temperature range of minus 40.degree. C. to 85.degree. C.
Yet another object of the invention is the provision of a microprocessor furnace control which is of relatively low cost, reliable and one which results in improved relay contact life.